1. Field of the Invention
The present invention relates generally to optical discs and optical disc readers. In particular, the invention relates to the use of standard optical disc drives, and slightly modified drives, to permit discriminable acquisition of a variety of different types of signals from an optical disc. The optical discs in such use include optical bio-discs discs having encoded information as well as investigational structures or features that are deposited on external or internal surfaces of the disc.
2. Description of Related Art
Commonly assigned, co-pending U.S. patent application Ser. Nos. 09/183,842 and 09/311,329 describe methods and apparatus for detecting operational and investigational structures on one or more surfaces of an optical disc assembly. Some of the methods and apparatus discussed in these applications detect investigational structures by physically modifying certain processing circuits in the optical disc drives. As an alternative to these and similar approaches, the present invention is directed to utilizing a principal advantage of relying on standard optical disc readers for laser microscopic detection. This advantage includes the ubiquitous distribution of such drives in the current consumer environment. Therefore, it would be desirable to provide methods and apparatus for detecting operational and investigational structures on an optical disc assembly without having to physically modify the processing circuitry herein.
To provide some background for further discussion of the present invention, relevant features of a conventional optical disc reader and optical disc are described briefly in connection with FIGS. 1–7. For purposes of preliminary introduction these figures will be briefly described. FIG. 1 is a cross-sectional view of typical single-layer CD or CD-like disc and a schematic representation of a reader associated therewith. FIG. 2 is a side cross-sectional view of the disc shown in FIG. 1 at greater magnification. FIG. 3 is a perspective view of the surface of a CD-R disc with wobble grooves. FIG. 4 is a schematic representation of an optical disc detector and associated electronics that use three beams for tracking, focusing, and reading. FIG. 5 is a plan view that illustrates the position of beams from a typical three-beam pickup relative to a track on an optical disc. FIG. 6 is a block diagram of a known optical disc reader. And, FIG. 7 is a functional block diagram of a conventional digital signal processing circuit.
More particularly now, FIG. 1 depicts the reader's optical pickup or objective assembly 10 and a conventional CD-type optical disc 11 with a light path indicated as dashed lines. For clarity, FIG. 1 depicts a minimal complement of the optical pickup components. FIG. 2 provides a side cross-sectional enlarged view of disc 11 in the same orientation relative to the incident light.
With reference to FIGS. 1 and 2, the conventional optical pickup 10 includes light source 19, lenses 12, 13, and 14, beam splitter 15, quarter wave plate 20, and detector 18. Light source 19 is placed at a focal point of a collimator lens 12 that normally has a long focal distance. Collimator lens 12 makes the divergent light rays parallel. A monitor diode (not shown) may be used to stabilize the laser's output. Light source 19 may be a laser, LED, or laser diode, although the present invention may be implemented on a non-coherent light system as well.
A conventional optical design used for three-beam pickup typically uses two secondary beams for tracking. To generate these beams, light from source 19 passes through diffraction grating 17, which is a screen with slits spaced only a few laser wavelengths apart. As the beam passes through the grating, the light diffracts; when the resulting collection is again focused, it will appear as a single, bright, centered beam with a series of successively less intense beams on either side. It is this diffraction pattern that actually strikes the disc.
A conventional three-beam pickup uses the center beam for reading data and focusing and two secondary beams for tracking only. In this design, the beams are spatially linked because they are the result of a single diffracted laser beam. By contrast, a one-beam pickup accomplishes all of these tasks using a single beam.
Polarization beam splitter 15 (PBS) directs the light to a disc surface and then directs the reflected light to the photodiode sensor 18. PBS 15 normally includes two prisms with a common 45° face acting as a polarizing prism. Collimator lens 12 preferably follows PBS 15. The light then passes through the quarter-wavelength plate 20, which is an anisotropic material that rotates the plane of polarization of the light beams. Light that has passed through quarter-wavelength plate 20 and that has been reflected from disc 11 back again through quarter-wavelength plate 20 will be polarized in a plane at right angles to that of the incident light. Because PBS 15 passes light in one plane, (e.g., horizontally polarized) but reflects light in the other plane (e.g., vertically polarized), PBS 15 deflects the reflected beam toward sensor 18 to read the digital data.
The final piece of optics in the optical path to disc 11 is objective lens 13, which is used to focus the beams onto the disc data surface, taking into account the refractive index of the light-proximal polycarbonate substrate 112 of disc 11. Objective lens 13 focuses the light into a convergent cone of light, or light spot. The convergence is a function of the numerical aperture of the lens.
The data encoded on disc 11 now determines the fate of the laser light. In a regular CD, when the light spot strikes a land, the smooth interval between two pits, light is almost totally reflected. When it strikes a pit with a depth of about a quarter wavelength of the light, diffraction and cancellation due to interference cause less light to be reflected. All three intensity-modulated light beams return through objective lens 13, quarter-wavelength plate 20, collimator 12, and PBS 15. Finally, these beams pass through singlet lens 14 and an astigmatic element 16, which may be a cylindrical lens, to introduce astigmatism in the reflected light beam en route to photodiode 18.
As shown in greater detail in FIG. 2, CD-type disc 11 includes three layers from the light-proximal surface to the light-distal surface. By convention, disc layers are numbered upwards from the light-proximal surface to the light-distal surface. These layers include the transparent substrate 112, a reflective layer 114, and a protective layer 116. Transparent substrate 112 makes up most of the thickness of a typical CD-type disc, as measured along the optical axis, and provides both optical and structural features necessary for disc operation.
Transparent substrate 112 is typically impressed or embossed with a spiral track that starts at the innermost readable portion of the disc and then spirals out to the outermost readable portion of the disc. In a non-recordable disc (e.g., pre-recorded), this track is made up of a series of embossed pits, each typically having a depth of approximately one-quarter the wavelength of the light that is used to read the disc. The pits have varying lengths. The length and spacing of the pits is employed as the mechanism for encoding the data.
With reference now to FIG. 3, the spiral groove in a recordable disc contains a dye rather than pits. A typical recordable disc includes a spiral groove having a characteristic shape along the length thereof. This type of groove is known as a “wobble groove,” and is formed by a bottom portion having undulating or wavy sidewalls. A raised or elevated portion separates adjacent grooves in the spiral. Such a wobble groove may then include embossed portion 110 and groove portion 118 as shown in FIG. 3. Embossed portion 110 and groove portion 118 are similar to the wobble groove found on a standard recordable CD.
Referring now to FIG. 4, the exemplary detector 18 and its associated electronics are described in more detail. Detector 18 typically includes a central detector 25, and can be bordered by additional side detector elements 26 and 27. Central detector 25 may be split into multiple detector elements (e.g., four), represented as A, B, C, and D. Detector elements A, B, C, and D (sometimes collectively referred to as a “quad detector”) each provide an electrical signal indicative of the intensity of the reflected light beam striking that element.
The sum of the signals from the quad detector 25, e.g., A+B+C+D, provides a radio frequency (RF) signal 50, also referred to as a high frequency (HF), quad-sum, or sum signal. As used herein the notation “A+B” indicates the sum of the signals from detector elements A and B. The HF signal 50 (i.e., RF, quad-sum, or sum signal) is typically demodulated to recover data recorded on the optical disc.
Various pairs of the signals from detector elements A to F are also combined to provide feedback signals for tracking and focus control. For example, a tracking signal 52 (e.g., tracking error or TE signal) is obtained from the difference between the E and F signals, (i.e., E−F). A focus error (FE) signal 54 may be obtained from the difference between the A+C and B+D signals.
Typically, such processing is performed by analog circuitry in combination with one or more integrated circuit chips. Often, the circuitry takes the form of a special chip set or a single ASIC (application-specific integrated circuit) chip.
The circuitry of FIG. 4 is just one example of circuitry that provides focus and tracking error signals in an optical disc player. Numerous methods are known for providing these signals. For example, a focus error signal may be obtained by the critical angle method, described in U.S. Pat. No. 5,629,514 or the Foucault and astigmatism methods, described in The Compact Disc Handbook by Pohlmann, A-R Editions, Inc. (1992) both of which are incorporated herein by reference in their entireties. Similarly, tracking error signals may be obtained using the single beam push-pull or three beam methods described in The Compact Disc Handbook or the differential phase method described in U.S. Pat. No. 5,130,963, which is incorporated herein by reference in its entirety, or the single beam high frequency wobble method.
With reference now to FIG. 5, a CD drive typically uses a three-beam pickup, in which the light beam is split into three beams, a main beam 21 and two tracking beams 23. The main beam 21 is focused onto the surface of an optical disc so that it is centered on a tracking structure, whereas the tracking beams 23 fall on opposite sides of the tracking structure. Main beam 21 is shown centered on track 24 (as defined by pits 22), with tracking beams 23 falling on opposite sides of track 24. By design, the three beams are reflected from the optical disc and directed to detector 18 (FIG. 4) so that main beam 21 falls on the quad detector, and tracking beams 23 fall on sensor elements E and F.
FIG. 6 is a generalized block diagram of an illustrative chip set 30 for a typical optical drive system. Although the chip sets for CD, CD-R, and DVD drives can be somewhat different from one another, it will be understood that the system shown in FIG. 6 is meant to generically represent all types of optical drives, and that a detailed understanding of the differences between the chip sets is not necessary to practice the present invention.
The HF signal 50, obtained from summing the signals from detector elements A, B, C, and D, is normally processed to extract whatever data is recorded on the optical disc. First, analog HF signal 50 is conditioned, with normalization and equalization performed. Next, analog signal 50 is converted to a digitized signal including a serial stream of digital data referred to as channel bits. The channel bit stream is then demodulated according to the modulation standard used for the type of optical disc being read. For example, it is common for CD-type discs to use eight-to-fourteen (also denominated “eight-of-fourteen”) modulation (EFM) wherein a data byte, or eight data bits, is encoded into fourteen channel bits. There are three merging bits between each group of fourteen channel bits. Thus, when reading a CD-type optical disc, seventeen channel bits are read from the optical disc, the merging bits are discarded, and the remaining fourteen bits are decoded, or demodulated, to obtain the original data byte. The data bytes themselves are grouped into blocks, which are further processed to reduce the effects of disc defects, such as scratches on the disc surface.
HF signal 50 from detector 18 (FIG. 4) may be converted to a square wave signal 51 by comparator 31, which provides a high output when HF signal 50 is above a threshold level, and a low output when HF signal 50 is below the threshold. Digital signal processing circuit (DSP) 32 then samples the resulting square wave signal 51 to determine the value of each channel bit. DSP 32 further demodulates the channel bits to extract the data bytes that are then grouped into blocks and processed to correct errors that may have occurred. Memory 33a provides temporary storage for the data, as it is being processed by DSP 32 and assembled into blocks.
Servo block 34 analyzes the tracking error (TE) signal 52 (or a wobble tracking error (WTE) in a DVD or CD-R system) and provides a tracking control signal to the tracking mechanisms to ensure that the pickup assembly maintains proper tracking. Similarly, a focus control signal 53 is provided based on focus error (FE) signal 54. DSP 32 provides an indication of the data rate of HF signal 50, which is used by servo block 34 to provide a speed control signal 55 to the spindle motor (not shown) of the optical disc drive.
In an audio CD player, after processing by DSP 32, each data block is sent to audio reproduction circuitry not shown in FIG. 6. However, in some data storage applications, each data block may contain additional error detection codes (EDC) and error correction codes (ECC). EDC/ECC circuitry 35 typically uses the EDC and ECC codes to increase the integrity of the data block by detecting and correcting errors not already corrected by DSP 32. Memory 33b, which may be combined with memory 33a, provides temporary storage for data blocks being processed by EDC/ECC circuitry 35. Finally, the data blocks are transferred from the optical disc player to host 37 by means of interface circuitry 36. Although an ATAPI interface is shown, it will be understood that other interfaces, such as SCSI, Firewire, or Universal Serial Bus (USB) and the like could also be used.
A controller 38 coordinates the operation of the various components of chip set 30, for example, by coordinating the transfer of data blocks between DSP 32 and EDC/ECC circuitry 35. Controller 38 also keeps track of which data block is being read and may keep track of various parameters indicative of the operational status of the optical disc reader.
Program memory 39 contains program code for the operation of controller 38. In many optical disc reader chip sets, program memory 39 may also contain program instructions for DSP 32 or EDC/ECC circuitry 35. This is advantageous for manufacturers in that the operation of the disc drive may be changed by altering the program code in program memory 39. For example, a newly developed method of modulating or encoding data on an optical disc may be accommodated by changing program memory 39.
FIG. 7 is a functional block diagram illustrating the signal processing that occurs within DSP chip 32 when configured in a conventional manner. As shown, DSP 32 performs several functions. For example, DSP 32 typically normalizes and/or equalizes the HF signal (block 40); converts the normalized HF signal from the analog-to-digital (block 42); demodulates and decodes the resulting EFM signal (block 44); performs some type of error checking procedure (e.g., using Cross-Interleaved Reed-Solomon Code “CIRC” block 46); and provides the resulting signal to an output interface (block 48) for communication with the host data bus 37 (FIG. 6). Examples of commonly used DSP chips that perform some or all of these functions include the SAA 7210, SAA 7220, and the SAA 7735, available from Philips Electronics Corporation, Eindhoven, Netherlands.
While the foregoing description is sufficient for a basic understanding of the present invention, there are numerous alternative designs and configurations of an optical pickup and associated electronics which may be used in the context of the present invention. Further details and alternative designs are described in Compact Disc Technology, by Nakajima and Ogawa, IOS Press, Inc. (1992); The Compact Disc Handbook, Digital Audio and Compact Disc Technology, by Baert et al. (eds.), Books Britain (1995); CD-Rom Professional's CD-Recordable Handbook: The Complete Guide to Practical Desktop CD, Starrett et al. (eds.), ISBN:0910965188 (1996); all of which are incorporated herein in their entirety by this reference.